Bone repair material

ABSTRACT

Sliceable bone repair material is a porous block-shaped scaffold containing a hydrogel, wherein the hydrogel is formed by Michael type addition of at least two precursor molecules. Said scaffold is made of a synthetic ceramic material and has interconnected macropores having a diameter above 100 μm. In addition said scaffold has a total porosity of 60 to 80%. The total volume of the hydrogel is smaller than the total volume of the interconnected macropores.

FIELD OF THE INVENTION

The present invention relates to a sliceable bone repair materialcomprising a porous block-shaped scaffold containing a hydrogel. Saidhydrogel is formed by Michael type addition of at least two precursormolecules. The scaffold is made of a synthetic ceramic material,comprises interconnected macropores and has a total porosity of 60 toless than 80%.

BACKGROUND

The repair of bone defects can be facilitated by placing a bone repairmaterial as a temporary substitute in the defect site, where a loss ofnatural bone has occurred. The bone repair material is meant toselectively promote and guide the regeneration of natural bonestructures.

Both naturally-derived and synthetically-produced bone repair materialshave been used to repair such defects. Naturally-derived materialsinclude grafts made from bones. The bone may be harvested directly fromthe patient, as in autograft-based procedures, or it may be harvestedfrom a suitable donor, surrogate, or cadaver, as in allograft- orxenograft based procedures. However, autograft bone implant proceduresare costly and cause additional discomfort for the patients, as theytypically require an additional surgery for harvesting the graftmaterial, which may cause significant morbity at the donor site.Autografts may also show pronounced resorption making the outcome of theaugmentation unpredictable. Allogenic bone repair materials also unifyosteoconductive and osteoinductive properties, but their origin raisespossible pathogenic transfers and ethical issues. Similar concerns arebrought up against xenogenic graft materials.

Alternatively, naturally-derived bone repair materials may be replacedby a completely synthetic bone repair material, which contains noorganic residues. In contrast to naturally-occurring bone repairmaterials, synthetic bone repair materials are often lessosteoconductive and hardly osteoinductive. Nevertheless, much researchhas been and still is directed toward improved synthetic bone repairmaterials.

In oral surgery and orthopedics, synthetic bone repair materials on ahydroxyapatite (HA) and/or tricalcium phosphate (TCP) basis are widelyused.

Depending on indications, they may be applied as granules orpre-fabricated blocks. U.S. Pat. No. 6,511,510 relates to a porousceramic material from calcium phosphates obtained by a sinteringprocess. The use of granular material allows treatment of a wide rangeof indications. For granular material, the ceramic block material isprocessed by steps such as rubbing, pounding and sieving afterwards (WO04/054633). Although the granular materials are applied to a wide rangeof indications in terms of size and area, their suitability to treatlarge bone defects is limited, because they tend to migrate and as aresult to be encapsulated. The augmented volume defined by the appliedgranules may collapse and fail to guide regrowth of the bone to itsoriginal dimensions. U.S. Pat. No. 7,012,034 describes a block-shapedbone augmentation material based on porous β-tricalcium phosphate.

Different approaches addressed the problem of providing a material withbone-like mechanical properties. In U.S. Pat. No. 6,994,726 a prostheticbone implant is made of a hardened calcium phosphate cement having anapatitic phase as a major phase, which comprises a dense corticalportion bearing the majority of load, and a porous cancellous portionallowing a rapid blood/body fluid penetration and tissue ingrowth.Alternatively, EP 1 457 214 discloses a block shape organic-inorganiccomplex porous article with a superposed skin layer made of a degradablepolymer with improved strength. The complex is mainly designed to beinserted between vertebral bodies.

To generally improve load bearing properties of bone repair materials,composite materials have been developed. EP 1 374 922 discloses abioresorbable structure for use in the repair of bone defects comprisinga porous bioceramic matrix of hydroxyapatite or tricalcium phosphate anda polymer disposed by compression molding therein. WO 97/34546 describesa ceramic block with a plurality of channels filled containing anenforcing bioresorbant polymer material. In order to improve theirregenerative potential, bone repair materials have been supplementedwith bone growth inducing agents. U.S. Ser. No. 10/271,140 (US2003/0143258A1) suggests a composite comprising demineralized bonematrix mixed with a stabilizing biodegradable polymer and a bone growthfactor.

In a typical periodontal surgical bone repair procedure an incision ismade in the gum tissue to expose a bone defect adjacent to a tooth root.Once the defect and root are debrided, a bone repair material, suspendedin a suitable carrier is placed. The gum tissue is then closed,maintaining the repair material in place. Optionally, a barrier materialmay be utilized to retain the repair formulation in contact with thedefect. Therefore, a bone repair material in periodontal surgeryrequires formulations that can be easily shaped to size and shape of thedefect. WO 2004/011053 suggests a formulation with a putty consistency.Similarly, EP 1 490 123 describes a kneadable bone replacement materialon a granular calcium phosphate and hydrogel basis. When applied to thedefect site, the formulation remains adhered thereto without migrationor excessive expansion. These concepts however, do provide for a solidbone substitute material.

US 2004/002770 discloses polymer-bioceramic structures for use inorthopaedic applications. Thereby a polymer is disposed in a porousbioceramic by compression molding.

WO 2009/007034 discloses a block-shaped scaffold and a hydrogel, whereinsaid scaffold comprises interconnected pores. Said interconnected poresare completely filled with a hydrogel. However, it has been found thatthe cells have difficulties to migrate into the scaffold material due tothe presence of the hydrogel.

SUMMARY OF THE INVENTION

It was a problem of the present invention to provide a bone repairmaterial which is easy to handle and suitable for treatment of largeoral bone defects, which has osteoconductive and osteoinductiveproperties.

It was found that a sliceable bone repair material wherein the totalvolume of the hydrogel that is evenly distributed within the scaffold issmaller than the total volume of the interconnected macropores showsexcellent osteoconductive and osteoinductive properties.

The bone repair material according to the present invention comprises aporous block-shaped scaffold containing a hydrogel. Said hydrogel isformed by a Michael type addition of at least two precursor molecules.The scaffold is made of a synthetic ceramic material and comprisesinterconnected macropores having a diameter above 100 μm. In addition,the scaffold has a total porosity of 60 to less than 80%. Within thecontext of the present invention the term scaffold “containing” ahydrogel means that the hydrogel is evenly distributed within themacropores of the scaffold. Preferably, the outer surface of thescaffold is essentially free of hydrogel.

In a preferred embodiment said hydrogel is a reconstituted hydrogel. Theterm reconstituted hydrogel (also called rehydrated hydrogel) stands fora hydrogel obtained after rehydration of a dried hydrogel (xerogel).

The hydrogel comprised in the bone repair material according to thepresent invention has in its hydrated or reconstituted (rehydrated) formpreferably a water content of more than 80% by weight, preferably ofmore than 90% by weight, most preferably between 90 and 98% by weight.In a most preferred embodiment of the present invention the hydrogelwith a PEG concentration of about 5% has a water content of 95% and ahydrogel with a PEG concentration of about 8% has a water content ofabout 92% by weight.

A dried hydrogel, also called xerogel, is the dry form of a hydrogel.Such a dry form of a hydrogel may be obtained by dehydrating a hydrogelunder vacuum.

The equilibrium concentration is the ratio between the mass of the freeprecursor molecules and the mass of the swollen hydrogel at equilibrium.A swollen hydrogel at equilibrium is the cross-linked network ofprecursor molecules which is hydrated or reconstituted (rehydrated) withthe amount of liquid it spontaneously takes up under physiologicalconditions (salt concentration, for example PBS buffer, and temperature,37° C.). The water content of the hydrogel is dependent on the affinityof the polymer, that is PEG, and the density of the crosslinking pointsof the hydrogel. A swollen PEG hydrogel has preferably a water contentof more than 90% by weight.

If the concentration of the precursor molecules in the bone repairmaterial is clearly lower than the equilibrium concentration, that is80% (90%), almost no crosslinking occurs. Due to the method which isdisclosed below, it is possible to increase the concentration of theprecursor molecules by water evaporating, and therefore, allowing theprecursor molecules to crosslink. Best results were obtained for bonerepair material, wherein the ratio of the total concentration of theprecursor molecules and the equilibrium concentration of the total ofthe precursor molecules in the swollen hydrogel is less 0.9 to 0.95,since this bone repair material showed good osteoconductive propertiesand had no tendency to break.

Dependent on the porosity of the material, the PEG content (that is thetotal concentration of the at least two precursors) is adjusted in orderto obtain good results. In a preferred embodiment the ratio between thescaffold material and the total of the precursor molecules is greaterthan 21:1, preferably greater than 35:1, and most preferably greaterthan 60:1. Such a combination allows very good cell integration and anoptimal bone repair material stability. Optimal results are achieved fora bone repair material having a scaffold with a porosity from 65 to 70%and a ratio between the scaffold and the total of the precursormolecules of more than 60:1 by weight.

Due to the ratio between the scaffold and the total of the precursormolecules in combination with the total porosity of the scaffold only 20to 80%, preferably 30 to 60, most preferably 20 to 45% of the totalvolume of the interconnected macropores is filled with cross-linkedhydrogel after rehydration, that is, the reconstituted hydrogel, whichresults in excellent osteoconductive and osteoinductive properties. Dueto the fact that 40 to 70%, preferably 55 to 80% of the total volume ofthe interconnected macropores is empty, that means is filled with air,blood and tissue fluids, the cells can easily migrate into the scaffoldmaterial and therefore nutrient and oxygen supply, neo-vascularisation,cell immigration, colonization and bone deposition are facilitated. Dueto the fact that such a good cell integration is reached it is possibleto use a scaffold material with a total porosity of 60 to less than 80%,preferably of 60 to 75%. Such a scaffold material is normally easier tohandle than scaffold material with a high total porosity.

Due to the new method for preparing the bone repair material accordingto the present invention, which is described below in detail, thecross-linked hydrogel is distributed within the block-shaped scaffold.The inner surface of the pores is more than 50%, preferably more than60% covered by a thin layer of the dried hydrogel, that is a xerogel,whereas interior space of the pore is not filled with hydrogel. Afterdrying the hydrogel only 1 to 15%, preferably 2 to 15%, most preferably2 to 5%, of the total volume of the interconnected macropores is filledwith the dried cross-linked hydrogel. The expression “dried” hydrogelmeans, that it contains only small amounts of water, preferably lessthan 10%, most preferably less than 5%. The xerogel has a high porosityand an enormous surface area along with a small pore size. The soobtained bone repair material has due to the low water content anexcellent shelf stability.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an electron microscopy picture of a block-shaped scaffoldwhich does not comprise a cross-linked polyethylene glycol hydrogel,

FIG. 2 shows an electron microscopy picture of a bone repair materialaccording to the present invention with a block-shaped scaffold with adried cross-linked polyethylene glycol hydrogel, that is a xerogel,

FIG. 3 shows an electron microscopy picture of a bone repair materialaccording to the present invention, prepared with 2% PEG hydrogel, and

FIG. 4 shows an electron microscopy picture of a bone repair materialaccording to the present invention, prepared with 5% PEG hydrogel.

DETAILED DESCRIPTION

Beside the good osteoconductive properties the bone repair materialaccording to the present invention has an also after rehydrationexcellent stability. In contrast thereto bone repair material havingcompletely filled pores, has a tendency to break upon rehydration orswelling. Especially bone repair material, wherein only 20 to 30% of thetotal volume of the interconnected pores is filled with the hydrated orreconstituted cross-linked hydrogel, showed no tendency to break at all.

The scaffold of the bone repair material according to the presentinvention has interconnected pores and is made of a synthetic ceramicmaterial. Preferably said synthetic ceramic material is a materialcomprising calcium phosphate. The synthetic ceramic material may be madeof apatite, tricalcium phosphate or a mixture thereof. Apatite andtricalcium phosphate (TCP) or combinations thereof, are efficient bonesubstitutes that enhance bone ingrowth. Eventually, the material getsresorbed and substituted by bone. Hydroxyapatite and β-tricalciumphosphate, and combinations thereof are especially preferred. Thesematerials can be manufactured with well defined reproduciblemorphologies with respect to grain size and porosity.

The scaffold material according to the present invention has a porousmorphology. Said scaffold material is a highly porous calcium phosphatewith interconnected pores of a size range that allows fast ingrowth ofnatural bone. A person skilled in the art can determine the pore volumeand the interconnectivity with micro CT. Methods to characterize calciumphosphate blocks with regard to the porosity have been described inBiomaterials, 2005 November; 26(31):6099-6105 and in Biomaterials 27(2006), 2776-2786.

For the preferred scaffold material according to this invention thetotal porosity lies in the range of 60 to less than 80%, preferably from60 to 75%, most preferably of 65%. Porosity is the percentage of voidspace per volume unit of scaffold material. Specific surface density isdefined as the scaffold surface per scaffold volume. The preferredscaffold material according to this invention has a specific surfacedensity of at least 20/mm, more preferably above 30/mm. The materialwill be integrated in newly formed bone and will eventually be degradedand replaced by natural bone. In the present invention the term porositymeans only open pores, e.g. pores which are accessible from outside.Closed pores do not fall within said definition.

The expression the total volume of the interconnected macropores meansthe void space of all macropores. Typically, the void space of allmacropores is measured by mercury intrusion porosimetry according to ISO15901-1.

The porous structure may be obtained by various processes. Usually apowder is suspended in an aqueous solution to result in a slurry. Toform a porous structure, a pore forming agent may be added.Alternatively a sponge-like polymeric matrix with a determined porestructure or an aggregate of spherical objects is coated with theslurry. After drying the slurry, the ceramic material undergoes asintering process at high temperatures between 800° C. and 1300° C.,depending on grain size and nanoporosity desired. During sintering thepore forming material is burned out and a porous ceramic scaffoldremains. Depending on the process and the pore forming agent ormaterial, the porosity of the block-shaped material can be adjusted toresult in a desired distribution and interconnectivity of pores ofvarious sizes. They can be classified as nanopores (diameter below 1μm), micropores (diameter between 1 and 100 μm) and macropores (diameterabove 100 μm). For the purpose of tissue regeneration, a substantialamount of interconnected micropores and macropores is desired in orderto allow cells to migrate into the scaffold material. Preferably morethan 90%, most preferably 95%, of the volume of the pores of theblock-shaped scaffold are micropores and macropores. Micropores aremostly not connected to other micropores but to macropores. Their mainfunction is to change the surface structure and area.

In a preferred embodiment of the invention, the diameter of the poreslies between 0.05 and 750 μm. More preferably, the diameter of themicropores is between 1 and 100 μm and the diameter of the macropores isbetween 100 and 1000 μm. Most preferably, the diameter of the microporesis between 5 and 100 μm and the diameter of the macropores is between100 to 500 μm. The pores of the preferred scaffold material according tothis invention have a mean diameter between 100 and 750 μm, preferably300 and 600 μm. The preferred embodiment further has highlyinterconnected pores. The interconnectivity can be defined as connectivedensity (equivalent to the terms connectivity or interconnectedness) asdescribed in Bone, 1993 March-April; 14(2): 173-82. The scaffoldmaterial according to this invention has a connectivity, which is above20 per mm³. In terms of connections per pore, which is equal to theratio of interconnectedness and the number of pores per volume, thescaffold material according to this invention has a connectivity perpore, which is at least 2, more preferably more than 3. As describedabove, the porosity does not need to be of random distribution, but maybe obtained by a highly repeated spacing structure such as tubuli. Atubular structure with a suitable stabilizing polymer may be preferred,if high mechanical strength is required.

In addition to the composition and porosity, a suitable architecture ofthe block-shaped ceramic scaffold material may further enhance boneregeneration and improve the handling properties. A first portion of theblock oriented to the remaining bone, which needs to be augmented,preferably has a cancellous structure with a high proportion ofmacropores, thereby facilitating the integration of bone tissue into theblock. A second portion of the block-shaped ceramic scaffold materialoriented to the surrounding soft tissue preferably has dense structurein order to reduce the risk of soft tissue ingrowth into the area ofbone augmentation. Therefore, the ceramic scaffold material subject tothis invention preferably is manufactured to contain a gradient in itsporosity and/or crystallinity and/or ceramic composition.

As mentioned above the bone repair material according to the presentinvention comprises a cross-linked polyethylene glycol hydrogel. Itcould be shown that the porous block-shaped scaffold comprising ahomogenously distributed cross-linked hydrogel has a waxy consistency,resulting in a bone repair material with excellent handling properties.The bone repair material is no longer brittle since the hydrogelstabilizes the pores, that is, it can be shaped with a scalpel, which isappreciated by the practitioner for fitting the block to the bone defectsite.

The cross-linked polyethylene glycol hydrogels in this invention arebased on the base catalyzed Michael type addition between a firstprecursor molecule and a second precursor molecule. In order to obtain across-linked network, at least one of the two precursors has more thantwo chains. The first precursor molecule has alpha, beta unsaturatedcarbonyl residues and the second precursor molecule has nucleophilicgroups. The resulting linkage between the first precursor molecule andthe second precursor molecule is an ester, which is unstable in contactwith water. Alternatively, non-hydrolyzable linkages may be formed ifmaleimid or vinylsulfone groups are used. The PEG-thiol can be replacedby peptides with two cysteine ends that are for example MMP sensitive toproduce non-hydrolyzable but enzymatically degradable gels The rate ofthe hydrolysis reaction depends on the temperature on the chemicalsurrounding of the ester (for example acrylate vs methacrylate onprecursor A, the number of methylene groups between O and S in precursorB) and the value of the pH, which is 7.4 in most tissues. Whensufficient bonds have hydrolyzed, the cross-linked network degrades orbreaks down. Therefore, the time of degradation of the network can beinfluenced by the number of hydrolysable bonds present per unit ofvolume. Such cross-linked polyethylene glycol hydrogels are describedfor example in EP 1 609 491.

The first precursor A preferably comprises a core which carries n chainswith a conjugated unsaturated group or a conjugated unsaturated bondattached to any of the last 20 atoms of the chain. The later statementdoes not necessarily mean that the chains comprise at least 20 atoms;the chains may also be shorter, in which case the conjugated unsaturatedgroup or a conjugated unsaturated bond is attached to an atom, which iscloser to the end of the chain. In a preferred embodiment saidconjugated unsaturated group or conjugated unsaturated bond is terminal.This means that said group is attached to the last atom of the chain,i.e. the one atom of the chain which is the furthest away from the coreof the precursor molecule A. The core of the first precursor A can be asingle atom such as a carbon or a nitrogen atom or a small molecule suchas an ethylene oxide unit, an amino acid or a peptide, a sugar, amultifunctional alcohol, such as pentaerythritol, D-sorbitol, glycerolor oligoglycerol, such as hexaglycerol. The chains are polyethyleneglycol chains. Beside the chains, the core of precursor A may beadditionally substituted with linear or branched alkyl residues orpolymers which have no conjugated unsaturated groups or bonds. In apreferred embodiment the first precursor A has 2 to 10 chains,preferably 2 to 8, more preferably 2 to 6, most preferably 2 to 4chains.

The conjugated unsaturated bonds are preferably acrylate groups,acrylamide groups, quinine groups, 2- or 4-vinylpyridinium groups,vinylsulfone groups, maleimide groups or itaconate ester groups offormula Ia or Ib

wherein R1 and R2 are independently hydrogen, methyl, ethyl, propyl orbutyl, and R3 is a linear or branched C1 to C1O hydrocarbon chain,preferably methyl, ethyl, propyl or butyl.

In a most preferred embodiment the precursor A has a core which carries2 to 6 polyethylene glycol chains, each chain having a terminal acrylategroup (also called a multi-arm PEG acrylate). The core of the precursorA is preferably a pentaerythritol moiety.

The second precursor B comprises a core carrying m chains each having athiol or an amine group attached to any of the last 20 atoms at the endof the chain. The later statement does not necessarily mean that thechains comprise at least 20 atoms; the chains may also be shorter, inwhich case the thiol or the amine group is attached to an atom, which iscloser to the end of the chain. In a preferred embodiment said thiol oramine group is terminal or attached to the second to last carbon atom ofthe chain. The expression “terminal” means that said group is attachedto the last atom of the chain, i.e. the one atom of the chain which isthe furthest away from the core of the precursor molecule A. For exampleit would also be possible to incorporate a cysteine residue into thechain. Preferably the thiol group is terminal. The core of the secondprecursor B can be a single atom such as a carbon or a nitrogen atom ora small molecule such as an ethylene oxide unit, an amino acid or apeptide, a sugar, a multifunctional alcohol, such as pentaerythritol,D-sorbitol, glycerol or oligoglycerol, such as hexaglycerol. The chainsare polyethylene glycol chains. In a preferred embodiment the secondprecursor B has 2 to 10 chains, preferably 2 to 8, more preferably 2 to6, most preferably 2 to 4 chains.

In a most preferred embodiment the precursor B has a core which carries2 to 4 polyethylene glycol chains, each chain having a terminal thiolgroup (also called a multi-arm PEG-thiol. The core of the precursor B ispreferably an ethylene oxide group or a CH₂-group.

The first precursor A compound has n chains, whereby n is greater thanor equal to 2, and the second precursor B compound has m chains, wherebym is greater than or equal to 2. The first precursor A and/or the secondprecursor B may comprise further chains which are not functionalized.

The sum of the functionalized chains of the first and the secondprecursor, that means m+n, is greater than or equal to 5. Preferably thesum of m+n is equal to or greater than 6 to obtain a well formedthree-dimensional network. Such molecules having a core and two or moreend groups are also referred to as multi-arm polymers.

Within the context of the present invention the expression the totalconcentration of the precursor molecules stands for the concentration ofthe total amount of precursor molecules A and of the total amount ofprecursor molecules B in a defined amount of water. Typically, the totalconcentration of the precursor molecules is indicated in weight %.

Beside the number of chains, their length is a crucial parameter toadjust thy hydrolysis/degradation time of the hydrogel network and themechanical properties of the bone repair material subject to thisinvention.

The number of atoms in the backbone (only carbon and oxygen atoms;hydrogen atoms are not counted) connecting two adjacent cross-linkingpoints is at least about 20 atoms, preferably between 50 and 5000 atoms,more preferably between about 50 and 2000 atoms, most preferably between60 and 1000 atoms, and ideally between 200 and 800 atoms. Across-linking point is here defined as a point in which 3 or morebackbone chains of the polymer network are connected.

In a preferred embodiment the first precursor has four polyethyleneglycol chains having each an acrylate group as terminal group and amolecular weight of 15 kDa. The second precursor has four polyethyleneglycol chains having each a thiol group as terminal group or attached tothe second to last carbon atom of the chain and a molecular weight of 2kDa. Such a hydrogel has an equilibrium concentration of about 8% byweight.

In a further preferred embodiment the first precursor has fourpolyethylene glycol chains having each an acrylate group as terminalgroup and a molecular weight of 15 kDa. The second precursor has twopolyethylene glycol chains having each a thiol group as terminal groupor attached to the second to last carbon atom of the chain and amolecular weight of 3.4 kDa. Such a hydrogel has an equilibriumconcentration of about 5% by weight.

The precursors forming the cross-linked polyethylene glycol hydrogel aredissolved or suspended in aqueous solutions. Since no organic solventsare necessary, only aqueous solutions and/or suspensions are present.These are easy to handle and do not require any laborious precautions asmight be the case if organic solvents were present. Furthermore, organicsolvents are an additional risk for the health of the staff and thepatients exposed to these solvents. The present invention eliminatessaid risk.

Said aqueous solution comprising the first and the second precursor canhave a neutral pH or can be acidic, such as diluted acetic acid. To saidsolution or suspension optionally a bioactive agent may be added.Optionally said solution or suspension has to be diluted further inorder to obtain a total polyethylene glycol concentration of less than8, preferably less than 5% by weight (depending on the precursor used),in order to make sure that no gelation occurs. In a next step the pH ofthe suspension or solution is increased to a range of 7.0 to 9.5,preferably 8.0 to 9.5. Said suspension or solution is added to thescaffold. Then the bone repair material is dried under vacuum. Sincewater evaporates the concentration of the precursors increases andreaches the minimal gelation concentration and therefore gelationoccurs. Typically, after drying under vacuum for 12 to 20 hours xerogelcontained in the bone repair material according to the present inventionhas a water content of less than 10%, preferably of less than 5%. Beforeuse said bone repair material, the xerogel has to be reconstituted bythe surgeon, by rehydrating the dried hydrogel (xerogel) with water orwith a buffered solution or soaking it for a few minutes in blood inorder to allow the hydrogel to swell. Such a reconstituted swelledhydrogel comprises typically about 5% to 8% PEG in total and has a watercontent of 92 to 98% by weight.

In a preferred embodiment the cross-linked hydrogel comprised in theporous block-shaped scaffold has a pH in a range of 8.0 to 9.5,preferably from 8.6 to 9.5. The expression the hydrogel “has” a pH offor example 8.6 means that the hydrogel is formed at a pH of 8.6 or isafter its formation brought to a pH of 8.6. That is, the pH is measuredeither a) in the buffered solution before adding said buffered solutionto the precursors; or b) directly after mixing the precursor beforecrosslinking occurs; or c) in the buffered solution before adding thebuffered solution to the dried product. Alternatively, the pH of thehydrogel can also be determined by calculation based on the projectedamounts and concentrations of the reagents in the reaction mixture.Preferably the pH of the hydrogel is measured right after mixing theprecursors, before soaking the scaffold with the mixture and beforedrying the bone repair material. Alternatively, the pH of the finalproduct is adjusted by rehydrating the dried bone repair material in abuffered solution having the desired pH.

Surprisingly, it has been found that in the case of a bone repairmaterial comprising a hydrogel having a pH in the rage of 8.0 to 9.5,preferably 8.6 to 9.5 the replacement of the hydrogel by newly formedbone is much faster than for hydrogels having a pH of about 7.6.Therefore, the bone regeneration takes place faster, leading to ashorter healing time. Although a difference of pH of about 0.5 to 1 mayseem fairly small, it has to be noted that the pH is based on alogarithmic scale and that, therefore, a difference in pH of 1corresponds to a difference in proton or hydroxide concentration by afactor 10.

The desired pH is achieved by addition of a basic catalyst or bufferregulating the pH. The concentration and the pH of the buffering agentmay also be used to adjust the speed of gelation. For example, it may beused to increase the speed of gelation for not so dense gels. The finalconcentration in the mixture that is added to the bone repair material,e.g. triethanolamine, is preferably between 0.005 mol/1 and 0.1 mol/l,preferably 0.01 mol/l.

In a further embodiment of the present invention the cross-linkedpolyethylene glycol hydrogel contained in the block-shaped scaffold isat the same time a matrix for sustained release of one or severalbioactive agents, which promote the osteoconductive and/orosteoinductive properties of the composite bone repair material. As usedherein, a bioactive agent is not limited by its origin or the way it isproduced and therefore can be extracted, synthetically or recombinantlyproduced and may have been subject to further processing orpurification, such as but not limited to, splicing, fragmentation,enzymatic cleavage or chemical modification. Examples of suitablebiologically active agents are BMPs (e.g. BMP-2, BMP-7), PTH, VEGF,Enamel Matrix Derivatives (EMD) or proteins contained therein, TGF-beta,IGF, Dentonin, and cell recognition sequences such as RGD, KRSR, etc.Most preferred the bioactive agent is BMP or RGD.

The bioactive agent may be covalently bound to the hydrogel, e.g. thiscan be achieved by a thiol moiety present in the bioactive agent, whichreacts with the conjugated unsaturated group or bond present inprecursor A upon mixing. A thiol moiety is present, e.g., in the aminoacid cystein. The bioactive agent is subsequently released from thehydrogel as the unstable ester linkage between the PEG and the bioactiveagent is hydrolyzed.

Alternatively, the bioactive agents may simply be entrapped orprecipitated into the bone repair material. The bioactive agent can beadded when mixing the other components of the composition. The bioactiveagent is then released by diffusion during and/or after degradation ofthe hydrogel. It is also possible to adsorb the bioactive agent on theblock-shaped scaffold prior to the soaking with the solutions comprisingthe first precursor A and the second precursor B.

The mechanical strength of the block shaped scaffold can be furtherenhanced by embedding one or more additional stabilizing polymers,fibrous or filamentous supplements such as carboxy methyl cellulose,alginates, xanthan gum etc.

In a preferred embodiment the bone repair material is prepared asfollows: In a first step a porous block-shaped synthetic scaffold isprepared comprising interconnected pores. Said scaffold has a totalporosity of 65 to less than 80%.

In a second step an aqueous solution comprising the first precursor A ismixed with an aqueous solution of a second precursor B, whereby theconcentration of the first precursor A and the second precursor Btogether is less than the equilibrium concentration of the swollenhydrogel, preferably less than 5% by weight, most preferably less than4% resulting in a mixture in which the total concentration of thereactive groups at the PEG molecules in the mixture is not high enoughto allow the cross-linking reaction to reach the point of gelation, i.e.a gel does not form. Thus, depending on their molecular weights, thetotal concentration and the individual concentrations of the firstprecursor A and the second precursor B may differ.

In a third step the block-shaped synthetic scaffold is soaked in themixture obtained in step two. Finally the bone repair material obtainedin step three is vacuumed, resulting in a gradual increase of theconcentration of the precursors, allowing the cross-linking reaction ofthe two precursors. The method is preferably carried out at roomtemperature. However, the temperature can be used to influence rate therate of evaporation and of the crosslinking. Preferably, the bone repairmaterial was dried slowly overnight under gradually increasing vacuum,in order to avoid foaming.

In a preferred embodiment the first precursor A has a concentration of0.5% to 4.5% by weight, preferably of 1.5 to 4.5% by weight, mostpreferably 2.0 to 2.5% by weight, and the second precursor B has aconcentration of 0.5% to 4.5% by weight, preferably of 0.5 to 3.0% byweight, most preferably 2.0 to 2.5%, before mixing them together, andthe total concentration of the first precursor A and the secondprecursor B in solution added to synthetic ceramic scaffold is at least2% by weight and less than 5% by weight, preferably less than 4% byweight.

Before use, the bone repair material has to be reconstituted by thesurgeon by soaking the dried bone repair material in water, in aqueoussaline solution, in a buffer solution or in blood. In the latter casethe cells and factors are incorporated directly into the block. However,due to the fact that the pores of the block are not completely filledwith the hydrogel, blood can also enter the bone repair material aftersoaking in water, aqueous saline solution, or a buffer solution.

In a preferred embodiment the surgeon is provided with a kit comprisingrepair material with the scaffold containing the xerogel (driedhydrogel) and a buffered solution, which is stored in a separatecontainer or compartment. Before use, the surgeon soaks the bone repairmaterial in the buffered solution in order to rehydrate the xerogel toreconstituted hydrogel in the bone repair material. The hydrogel isreconstituted in less than 5 minutes.

EXAMPLES Example 1 Gelation Experiments Loose Hydrogels

6.4 mg of 4-arm PEG with acrylate end groups (M_(n)=15 kDa) and 3.3 mgof linear PEG with propylthiol endgroups (M_(n)=3.5 kDa) were dissolvedin 64 μl of 0.04% aqueous acetic acid. To the solution 25 μl of 0.10 Mtriethanolamine were added, yielding a solution containing 9.8 wt % PEG.At 25° C. the solution yielded a soft, elastic hydrogel in 6.5 min.

3.3 mg of 4-arm PEG with acrylate end groups (M_(n)=15 kDa) and 1.7 mgof linear PEG with proplythiol end groups (M_(n)=3.5 kDa) were dissolvedin 69 μl of 0.04% aqueous acetic acid. To the solution, 25 μl of 0.1 Mtriethanolamine were added, yielding a solution containing 5.0 wt % PEG.At 25° C. the solution yielded an elastic hydrogel, softer than the 9.8wt % hydrogel in 16 min.

1.32 mg of 4-arm PEG with acrylate end groups (M_(n)=15 kda) and 0.69 mgof linear PEG with propylthiol end groups (M_(n)=3.5 kDa) were dissolvedin 72.4 μl of 0.04% aqueous acetic acid. To the solution, 25 μl of 0.1 Mtriethanolamine were added, yielding a solution containing 2.0 wt % PEG.At 25° C. the solution did not yield a hydrogel within the observationtime of 4 hours.

Example 2 Degradation Loose Hydrogels

A gel containing 9.8 wt % PEG was placed in PBS at pH 7.4 and 37° C.After 6 hours the gel had swollen to 2.2-fold its original weight. Afterthat, swelling continued at a slower pace and after 11 days the gel wascompletely dissolved.

A gel containing 5.0 wt % PEG was placed in PBS at pH 7.4 and 37° C.After 6 hours the gel had swollen to 1.1-fold its original weight. Afterthat, swelling continued at a slower pace and after 11 days the gel wascompletely dissolved.

Example 3 Gelation Experiments Denser Hydrogel

7.0 mg of 4-arm acrylate end groups (M_(n)=15 kDa) and 1.1 mg of 4-armPEG with propylthiol end groups (M_(n)=2.3 kDa) were dissolved in 68.4μl of 0.04% aqueous acetic acid. To the solution, 25 μl of 0.2 Mtriethanolamine/HCl buffer (pH 8.5) were added, yielding a solutioncontaining 8.0 wt % PEG. At 25° C. the solution yielded a firm hydrogelin 2 to 3 min.

Degradation Denser Hydrogel

The denser gel containing 8.0 wt % PEG was placed in PBS at pH 7.4 and37° C. After 6 hours the gel had swollen to 1.2 fold its original weightand after 28 days the gel was completely dissolved.

Example 4 Combination Bone Block and PEG Hydrogel 5% PEG HydrogelSolution

66.7 mg of 4 arm PEG with acrylate end groups (M_(n)=15 kDa) and 32.7 mgof linear PEG with propylthiol end groups (M_(n)=3.5 kDa) were dissolvedin 1173 μl of 0.04% aqueous acetic acid and 200 μl of water and 500 μlof aqueous 0.1 M triethanolamine solution were added.

3% PEG Hydrogel Solution

39.7 mg of 4 arm PEG with acrylate end groups (M_(n)=15 kDa) and 19.8 mgof linear PEG with propylthiol end groups (M_(n)=3.5 kDa) were dissolvedin 704 μl of 0.04% aqueous acetic acid and 920 μl of water and 300 μl ofaqueous 0.1 M triethanolamine solution were added.

2% PEG Hydrogel Solution

27.1 mg of 4 arm PEG with acrylate end groups (M_(n)=15 kDa) and 13.2 mgof linear PEG with propylthiol end groups (M_(n)=3.5 kDa) were dissolvedin 470 μl of 0.04% aqueous acetic acid and 1280 μl of water and 200 μlof aqueous 0.1 M triethanolamine solution were added.

1% PEG Hydrogel Solution

13.2 mg of 4 arm PEG with acrylate end groups (M_(n)=15 kDa) and 6.6 mgof linear PEG with propylthiol end groups (M_(n)=3.5 kDa) were dissolvedin 235 μl of 0.04% aqueous acetic acid and 1640 μl of water and 100 μlof aqueous 0.1 M triethanolamine solution were added.

Bone Block

Immediately after mixing, the 2.0 ml of hydrogel solution was applied toa cylindrical bone block (h=25 mm, Ø=14 mm) with a porosity of 71% whichabsorbed the liquid completely.

The filled block was placed in an exsiccator that was heated to 37° C.and the pressure was gradually reduced to 5 mbar over the course of 5hours, after which the block was left at 5 mbar and 40° C. for another15 hours. This procedure almost completely removed the water withoutcausing foaming and, in the process of water removal, allowed forefficient gelation.

Before addition of the hydrogel solution and after drying, the blockswere weighed. For all blocks, the increase in weight was found to bebetween 90% and 103% of the total weight of both PEG precursors.

It was observed that, upon prolonged incubation in cell medium (7 daysat 37° C.) some blocks broke, presumably due to the pressure of theswelling hydrogel on the thin calcium phosphate walls of the bone block.

The ratio between scaffold (bone block) and total precursor molecules isalso indicated.

Wt % Blocks PEG- PEG- Total Ratio PEG broken Scaffold acrylate thiolPrecursor scaffold/total in after 7 [mg] [mg] [mg] [mg] precursorhydrogel days 4134 66.7 32.7 99.4 41.6 5 20% 4244 39.7 19.8 59.5 71.3 33% 4262 27.1 13.2 40.3 106 2 0% 4387 13.2 6.6 19.8 222 1 0%

Each one of the blocks prepared with the 2% formulation, the 5%formulation, and without hydrogel were broken and the fracture surfaceswere studied with SEM. Both the blocks with 2% and 5% PEG hydrogelformulation showed the presence of sheets of xerogel lining the walls ofthe pores. In the 5% block (FIG. 4) these were thicker than in the 2%block (FIG. 3).

EDX (Energy-dispersive X-ray spectroscopy) analysis showed the absenceof C in areas without xerogel and the presence of C in the areas linedwith xerogel.

1. A sliceable bone repair material comprising a porous block-shapedscaffold containing a hydrogel, wherein the hydrogel is formed byMichael type addition of at least two precursor molecules, and whereinsaid scaffold is made of a synthetic ceramic material and comprisesinterconnected macropores having a diameter above 100 μm, said scaffoldhaving a total porosity of 60 to less than 80%, and the total volume ofthe hydrogel is smaller than the total volume of the interconnectedmacropores.
 2. Bone repair material according to claim 1, wherein thehydrogel is a reconstituted hydrogel of a xerogel.
 3. Bone repairmaterial according to claim 1, wherein the hydrogel has a water contentof more than 90% by weight.
 4. Bone repair material according to claim1, wherein 30 to 60 of the total volume of the interconnected macroporesis filled with said hydrogel.
 5. Bone repair material according to claim1, wherein the hydrogel has a pH from 8.0 to 9.5.
 6. Bone repairmaterial according to claim 1, wherein the first precursor molecule hasa core with 2 to 4 polyethylene glycol chains, each chain having aterminal acrylate group.
 7. Bone repair material according to claim 1,wherein the second precursor molecule has a core with 2 to 4polyethylene glycol chains, each chain having a thiol group which isterminal or attached to the second to last carbon atom.
 8. Bone repairmaterial according to claim 1 further comprising a bioactive agent. 9.Bone repair material according to claim 8, wherein the bioactive agentis selected from the group of RGD and BMP.
 10. A sliceable bone repairmaterial comprising a porous block-shaped scaffold containing a xerogel,wherein the xerogel is formed by Michael type addition of at least twoprecursor molecules, and wherein said scaffold is made of a syntheticceramic material and comprises interconnected macropores having adiameter above 100 μm, said scaffold having a total porosity of 60 toless than 80%, and the total volume of the xerogel is smaller than thetotal volume of the interconnected macropores.
 11. Bone repair materialaccording to claim 10, wherein the first precursor molecule has a corewith 2 to 4 polyethylene glycol chains, each chain having a terminalacrylate group.
 12. Bone repair material according to claim 10, whereinthe second precursor molecule has a core with 2 to 4 polyethylene glycolchains, each chain having a thiol group which is terminal or attached tothe second to last carbon atom.
 13. Kit comprising a bone repairmaterial according to claim 10 and a buffered solution, said bufferedsolution being stored in a separate container.
 14. Kit according toclaim 13, where in the buffered solution is between pH 8.0 and 9.5. 15.Kit according to claim 14, wherein the buffered solution is between pH8.6 and 9.5.
 16. Bone repair material according to claim 5, wherein thehydrogel has a pH from 8.6 to 9.5.